It is well known to those skilled in the art that the main ingredients of a nuclear fusion process are the nuclei of Hydrogen. In particular, its isotopes, deuterium and tritium (D-T). A fusion reaction consist of these light nuclei coalescing to form heavier elements with the release of energy. In order to coalesce or react these isotopes, referred to as the fuel, must be ionized to form a plasma. In the plasma state they have a net positive charge and normally repel one another. Therefore, before a fusion reaction can occur the repulsion force between the nuclei must be overcome by energetic collisions. However, even in energetic collisions there is a greater probability that the colliding nuclei will not fuse but will, instead, rebound elastically.
Thus, the plasma must be confined in a region where they will approach each other and collide many times until fusion eventually takes place. The temperature required for the collisions to be energetic enough for the fusion reaction to produce more energy than the plasma loses by radiation is of the order of 10.sup.8 .degree. K. This temperature is a threshold value or initiation temperature; and because of the energy losses such as radiation and particle losses due to instabilities which occur concurrently with the additions of heat energy, the critical temperature for sustaining the reaction has to be even higher than that required to initiate fusion.
The critical temperature requirment and the need for confining the plasma for a long enough time for an appreciable fraction of the fuel to burn precludes the use of material walls. Thus, for relatively low density plasmas, all feasible methods of containing the plasma rely on some form of magnetic confinement means. These low density plasmas require quite long confinement times because it is agreed by those skilled in the art that the product of density and confinement time must exceed a certain value before the reaction can be sustained. However, the longer the confinement time, the greater the energy losses due to the leakage of particles, to other instabilities, and to radiation which is greatly aggravated by the presence of any contaminates.
For many years the research effort on controlled thermonuclear fusion was dominated by problems associated with magnetic confinement. However, all attempts to achieve useful controlled fusion energy release by these methods have been unsuccessful. Therefore, recently, there have been attempts to solve the controlled thermonuclear fusion problem by producing a microsize nuclear explosion. To accomplish this it is necessary to use a small fuel target and to produce a small clean energetic trigger which is capable of producing the very high temperatures required to ignite a very small thermonuclear explosion in the small volume of dense D-T mixture within a very short time. This time is determined by the time required for the plasma to cool.
An ideal trigger would produce the necessary energy in a very short pulse which could be readily guided to the target and focused into the small volume of the target. The energy should be in a form such that it is totally absorbed by the small amount of target material. Preferably in a manner so that the nuclear fuel is heated uniformly. Thus, because of the ease with which short pulse length, high power laser beams can be guided to targets and focused into small volumes, they have been considered as a trigger. The basic idea of a laser-driven fusion device is to heat a small pellet or target containing a deuterium-tritium (D-T) mixture to ignition temperatures by the absorption of laser light in a time short compared to the time, T.sub.d, of its disassembly at the speed of sound in the material at the ignition temperature.
Of course, the D-T reaction time, T.sub.r, must be short compared to T.sub.d and the range of the 3.5 MeV alpha particles which are produced should be less than the radius of the target. These conditions cannot be met at solid D-T densities with lasers which can be expected to be developed in the foreseeable future. Therefore, the targets are designed so that the D-T fuel is compressed by a factor of 10.sup.3 to 10.sup.4 above its solid density by a compressive pressure which is the reaction to the outward momentum of an ablating outer region of heavy material about the target. Pressures of the order of 10.sup.12 atmospheric are estimated to occur during the ablation-implosion process in laser fusion concepts. However, to achieve these pressures the laser pulse or pulses must have a special form, that is, they must be tailored according to the target requirements and the target must be spherically radiated. Multiple laser beams can be produced and guided to targets so that the targets are spherically radiated. Special care must be taken so that the beam from each laser arrives at the target at the same time. Laser beams have been used to generate high temperature dense plasma from which a few (10.sup.4) neutrons have been obtained. However, the energy limit of currently available lasers imposes severe restrictions on their use for this purpose.
Numerous articles have been published which disclose the limitations of lasers for this purpose. A typical article is "LASER FUSION", Practical Power Plant May be Unattainable--Panel" Nuclear News, pages 79-80, May 1975. State of the art laser technology is noted as being inadequate for laser fusion.
Relativistic electron beams possess energies which are several orders of magnitude larger than the best laser beams. Because of this, they are receiving attention as a means of achieving substantial thermonuclear yields by compression and heating of small masses of D-T fuel. This, of course, is the electron beam analogue of the inertial confinement schemes that employ lasers. However, difficulties associated with the use of high energy electron beams stem from the requirements for focusing and guiding them to small targets; from their relatively long pulse lengths (tens of nanoseconds); and from their long energy deposition lengths in dense mixtures of T-D fuels. To overcome these problems, several prior art methods have been proposed which require the use of multiple electron beams and specially designed targets. For example, U.S. Pat. Nos. 3,892,970; 3,899,681 and T. G. Roberts et al, "An Electron Beam Initiated Fusion Neutron Generator," IEEE TPS, Vol. PS-2, pp. 257-260, December 1974, teach that the electron beams are to be delivered to the target simultaneously so that the target is radiated properly and hydrodynamic instabilities do not develop. Consideration of hydrodynamic stability requirements impose severe restriction on the design of electron beam imploded fusion targets. Thus, when several beams are used, a high degree of simultaneity is required to insure a sufficiently uniform implosion.